I've worked at stables that made their own feed from combining commercial bags of various foods. The base mix was trotter, sweetfeed(when we had it), corn, and rolled oats, all dumped together and stirred in a big garbage can. We added bran in the winter with plenty of hot water and let it cook before feeding.
BUT and this is a big one, you need to base your feed requirements on what your horse needs. We used this mix because we needed an all around combo. The drafts got no sweetfeed and double corn. I've seen College studies where they used only rolled oats and barley. So if you decide to make your own feed combination, make sure you talk with someone who understands horse nutrition and knows YOUR horse. The local CO Op may have some advise and may even mix their own grain.